This application discloses a process whereby metallic scrap can be decontaminated in a container by concurrently passing in the same direction both the scrap and a hot gas through the container transferring heat from the hot gas to the scrap.
In my application Ser. No. 771,846 now U.S. Pat. No. 4,264,060 I described some of the known art methods for decontaminating scrap. Briefly, these can be broken down as follows. Certain processes decontaminate scrap by passing the scrap through a rotary kiln which also contains within the interior of the kiln a burner which heats the interior of the kiln. Utilizing this type of process, the scrap within the kiln is subjected to direct impingement of the flame on the surface of the scrap. This is considered disadvantageous because those portions of the scrap which are subjected to the flame are heated beyond that point necessary to burn the scrap and in fact are heated to a point where oxidation or melting of the metallic scrap itself takes place.
In a different type of decontamination process the scrap is placed in either trays or in piles within a furnace. The scrap is heated indirectly by heating either the walls of the furnace or the gases within the interior of the furnace. This method has the advantage over the previously described method in that the surface of the scrap is not directly impinged upon by the flame; however, since the scrap is either located in a container or in a pile the scrap within the interior of the container or a pile in effect can only be heated by heat conducted by the scrap surrounding it. In order to heat the scrap within the interior of the pile sufficient to decontaminate it, the scrap on the exterior of the pile is heated to a point above that necessary to decontaminate it which results in surface oxidation. This process in effect is analogous to baking a roast in an oven. Before the middle is done, the outside is browned.
In a third process the scrap is passed through a rotating kiln and a hot gas is passed countercurrent to the scrap. A representative example of this process is that described in U.S. Pat. No. 4,010,935. In this process the scrap exiting the kiln is at its highest temperature and it is at this point that it is exposed to the maximum temperature gas. At the inlet end of the kiln the scrap is relatively cool such as at ambient temperature, and the gas exiting the inlet end of the kiln is cooler than the gas passed into the kiln at the scrap exit end of the kiln because it has lost heat to the scrap. To decontaminate the scrap the scrap must be raised to a temperature causing decomposition of the contaminants. Further, it must be held there for a time sufficient to ensure complete decomposition. Since the scrap exiting the kiln contacts the hottest temperature gas just prior to exiting the kiln either one or the other of the following takes place. If the scrap is only raised to the oxidation temperature at the moment it exits the kiln, it will not be held at this temperature for a period long enough to ensure complete oxidation of the contaminants. If the scrap obtains the proper oxidation temperature some time prior to when it exits the kiln, than if it remains in the kiln for a time period sufficient to ensure complete oxidation, because it will be further contacted with high temperature hot gas before it is discharged from the exit end of the kiln, it will be heated to a temperature greater than the optimum decontamination oxidation temperature and will be exposed to temperature which can cause metal oxidation.
Most of man's most useful metals are found on the earth in the form of ores. To convert these ores to the free metals energy must be expended. It is also possible to collect used metals such as scrap and recycle them or re-refine them into new starting stock for man's consumer products. With certain metals, aluminum being a representative example, conversion from the ore to the pure metal requires a greater energy expenditure than conversion of reprocessed or scrap metals back into basic feed stocks. In view of the necessity for energy conservation the reprocessing of scrap and used metals has become of paramount importance.